Nowadays, for reasons associated with comfort and safety, seats in vehicles are ventilated. For this purpose, both the driver's seat and other seats in a vehicle can be provided with ventilation ducts that lead from a fan located in the vicinity of the seat to one or several openings in the seat. Such openings are normally located in the seat cushion, but can also be located in the seat back. The fan can be arranged so that it either blows air or extracts air. In this way, ventilation of the surface of the vehicle seat is achieved, which in turn gives the driver or passenger sitting on the seat in question an increased feeling of comfort.
A problem with previously-known arrangements for forced ventilation of vehicle seats is the difficulty in obtaining an accurately controlled temperature at the surface of the seat. WO 02/06914 shows an arrangement for temperature control of a vehicle seat comprising a fan for ventilating the said seat.
This design does not, however, allow any precise control of the fan for obtaining as accurately controlled a temperature as possible at the surface of the seat.
In a general context, methods for controlling the speed of rotation and output of a direct current motor by means of pulse width modulation, abbreviated as PWM, are currently known. In pulse width modulation, the direct current motor is driven by a pulse train generated in an external control unit, which means that during the pulse train's positive live phases, the direct current motor is live, while during the dead phases of the pulse train, the direct current motor is dead. Thus, during a supply period, the direct current motor receives drive voltage from and including a rising edge up to and including a falling edge.
The best conditions have normally been achieved when the switching frequency of the pulse-modulated signal is considerably higher than the speed of rotation of the motor, whereby the output of a conventional direct current supply is obtained. A theoretical rule of thumb for obtaining the correct switching frequency is that the switching frequency must be higher than 1/Ta where Ta=L/R is the electrical time constant of the motor. Here L is the inductance of the motor and R is the internal resistance of the motor. Normally, switching frequencies are used that are within the approximate range 20 Hz-200 kHz, which can mean that audible dissonance from the motor arises or that electrical interference arises caused by the pulse width modulation.
For brushless direct current motors, a position sensor is required, which informs the driving electronics about the phase angle of the rotor magnets in relation to the field magnets. The drive electronics apply current to the windings on the basis of the output signal from this sensor. This is called electronic commutation. A so-called Hall detector is normally used as the sensor for obtaining this commutating output signal.
The pulse width modulated supply is currently implemented in such a way that a direct current motor is supplied with a pulse train with a fixed frequency generated in an external control unit. By feeding back the speed of rotation to the control electronics, the width of the signals in this pulse train can be varied in such a way that the motor receives voltage for a shorter or longer time during each supply period. In this way, the speed of rotation and output of the direct current motor are proportional to the pulse width of the supply pulse.
U.S. Pat. No. 6,381,406 shows how a direct current motor for cooling electronic equipment is connected to a pulse width modulated signal. The direct current motor emits an output signal in the form of a feedback signal “TACH” that corresponds to the speed of rotation of the motor. This output signal “TACH” is compared with a reference signal “SYNCH” that corresponds to the required speed of rotation. When “TACH” deviates from “SYNCH”, the pulse width is adjusted so that “TACH” and “SYNCH” conform with each other.
Patent DE10009128 C1 shows a fan for the ventilation of vehicle seats, where a control unit integrated in the fan unit is supplied with control signals from an external control unit. In turn, the control unit integrated in the fan unit supplies the fan motor with drive voltage on the basis of the control information received. This drive voltage for the fan motor comes in to the control unit integrated in the fan unit via a separate supply lead for drive voltage.
A disadvantage of previously known general technology for direct current motors is that a large amount of cabling is required in order to achieve both the transmission of signals and the transmission of power. This is also associated with disadvantages associated with the need to use pulse width modulation with a high pulse frequency in order to avoid audible dissonance that originates from the pulse width modulated supply, as this gives rise to problems with electromagnetic interference, commonly given the abbreviation EMC (ElectroMagnetic Compatibility), caused by the high frequency of the switched signal.